Active matrix displays are being developed which utilize liquid crystals or electroluminescent materials to produce high quality images. These displays are expected to supplant cathode ray tube (CRT) technology and provide a more highly defined television picture. The most promising route to large scale high quality liquid crystal displays (LCDs), for example, is the active-matrix approach in which thin-film transistors (TFTs) are co-located with LCD pixels. The primary advantage of the active matrix approach using TFTs is the elimination of cross-talk between pixels, and the excellent grey scale that can be attained with TFT-compatible LCDs.
Flat panel displays employing LCDs generally include five different layers: a white light source, a first polarizing filter that is mounted on one side of a circuit panel on which the TFTs are arrayed to form pixels, a filter plate containing at least three primary colors arranged into pixels, and finally a second polarizing filter. A volume between the circuit panel and the filter plate is filled with a liquid crystal material. This material will alter the polarization of light in the material when an electric field is applied across the material between the circuit panel and a ground affixed to the filter plate. Thus, when a particular pixel of the display is turned on, the liquid crystal material adjusts light being transmitted through the material so that it will pass through the second polarizing filter.
The primary approach to TFT formation over the large areas required for flat panel displays has involved the use of amorphous silicon which has previously been developed for large-area photovoltaic devices. Although the TFT approach has proven to be feasible, the use of amorphous silicon compromises certain aspects of the panel performance. For example, amorphous silicon TFTs lack the frequency response needed for large area displays due to the low electron mobility inherent in amorphous material. Thus the use of amorphous silicon limits display speed, and is also unsuitable for the fast logic needed to drive the display.
Owing to the limitations of amorphous silicon, other alternative materials include polycrystalline silicon, or laser recrystallized silicon. These materials are limited as they use silicon that is already on glass which generally restricts further circuit processing to low temperatures.
Thus, a need exists for a method of forming high quality TFTs, driver circuits, and electrodes at each pixel of a panel display having the desired speed and providing for ease and reduced cost of fabrication.
The present invention relates to panel displays and methods of fabricating such displays using thin-films of single crystal or essentially single crystal silicon in which transistors are fabricated to control each pixel of the display. These methods are used to fabricate transmissive displays such as liquid crystal projection displays, or alternatively, for emissive displays including electroluminescent (EL) displays, both of which can be used for a variety of applications including head mounted displays.
For a preferred embodiment, the thin-film or transistor array is transferred onto an optically transmissive substrate such as glass or transparent plastics. The transfer process typically involves attaching a transparent substrate to the circuit with an adhesive and removing the semiconductor substrate from the silicon-on-insulator (SOI) structure on which the SOI structure containing the circuit is formed. In this embodiment, the thin-film single crystal silicon is used to form a pixel matrix array of thin-film transistors which actuate each pixel of an LCD. CMOS circuitry that is highly suitable for driving the panel display can be formed in the same thin-film material in which the transistors have been formed. The circuitry is capable of being fully interconnected to the matrix array using thin-film metallization techniques without the need for wires and wirebonding.
The pixel electrodes formed in the active matrix display are made from a transparent conductive material such as indium tin oxide, or other metal oxides such as titanium dioxide or zinc oxide. Conductive nitrides, such as altiminium nitride, for example, can also be used. These electrodes can, in certain preferred embodiments, be formed prior to transfer of the circuit onto a transparent substrate. Alternatively, in other preferred embodiments, the pixel electrodes can be formed after transfer of the active matrix circuit onto a transparent substrate. In these latter embodiments vias are formed through the insulating layer on which the transistor circuits are formed to conductively connect the pixel electrodes to their respective switching transistors. This also permits the electrodes to be fabricated over the transistor circuits.
Another preferred embodiment of the invention includes the fabrication of color filter elements to form a color display. The color filter elements are preferably formed prior to transfer and are thus fabricated on the same side of insulating layer as the pixel electrodes, or alternatively, on the opposite side of the insulating layer as the pixel electrodes, or in a third alternative, are formed on the optically transmissive substrate prior to transfer of the circuit onto the substrate. The color filter elements can include blue, green and red regions of polyimide or other suitably pigmented material formed in a pattern that is aligned with the pixel electrode array in the resulting display device. Alternatively, a substractive color display can be formed using color filter elements having cyan, magenta and yellow pigments.
This structure for the color filter system permits the color filter elements to be placed in close proximity to the pixel electrodes. For transmissive systems in which the light is not highly collimated and the pixel size is small, that is, having a pitch between 10 microns and 100 microns and preferably in the range between 10 microns and 30 microns, it is desirable to minimize the distance between the filter element and the corresponding pixel electrode to reduce the propagation of off-axis light from a given pixel electrode through the filter element of a neighboring pixel. In the present system, the color filter elements are placed directly on the pixel electrode material, or alternatively, are positioned at a distance in the range of 1-10 microns from the pixel electrode, preferably in the range of 1-4 microns.